Compared to every other browser I've used, Chrome, Edge, and Firefox, Vivaldi is the only one where facebook looks different. Normally your friends list and some game stuff would appear in a panel on the entire right side but instead it just appears as a small chat window. The only time other browsers will display facebook in the same way i see it in Vivaldi is if the browser width is below a certain threshold. Example, Microsoft Edge vs Vivaldi, same screen width. http://i.imgur.com/L4ZqMAm.png
After coming across this same issue on engadget and another site, I believe V has some artificial limit/bug where it wont allow a page to be wider than apx 1000 px
You can get back the same layout as Chrome, Firefox etc. on FaceBook by hiding the Vivaldi panel (Not minimizing the Panel but hiding it completely).
Hope this can help you.